SP1 and adult T-cell leukemia/lymphoma: These transcripts are initiated from the 3′ LTR; and the hbz-sp1 mRNA which codes for the most abundant isoform (HBZ-SP1) present in ATLL cells (Fig. 3a), includes exon 1 (nts 1–367) spliced to an acceptor site at position 1767 on the minus strand.